Downhole release joint

ABSTRACT

A downhole apparatus permitting separation of one part of downhole equipment from another part thereof includes a first part ( 4 ) connected to a second part ( 6 ) by connecting means which limits axial and rotational movement between the parts. The connecting means includes a connecting member ( 22 ) having first and second ends connected respectively to the first and second parts ( 4, 6 ). The connection of the first and second ends of the connecting member ( 22 ) is such as to limit axial and rotational movement relative to the respective first and second parts ( 4, 6 ). The connecting member ( 22 ) is provided between the first and second ends with a region of weakness in the connecting member provided by weakening means ( 28 ), and with means for minimizing stress generated in the connecting means at the region of weakness when torque is transmitted in use between the first and second parts ( 4, 6 ) via the connecting member ( 22 ).

[0001] The present invention relates to downhole apparatus permittingseparation of one part of downhole equipment from another part thereof.

[0002] It is not uncommon for a drill bit to become stuck inhole duringdownhole oil and gas drilling operations. In order to allow retrieval ofa downhole drill string when a drill bit becomes jammed, it is known toprovide a drill string with an emergency release joint immediatelyuphole of the drill bit. During normal operation, the release jointtransmits torque from a motor to the drill bit. However, in the eventthat the drill bit becomes jammed to the extent that axial androtational movement of the drill bit is not possible, the drill bit maybe separated from the remainder of the drill string by virtue of therelease joint. The remainder of the drill string may then be movedaxially uphole so that specialist retrieving equipment may be run to thedrill bit in a fishing operation.

[0003] Although the prior art release joints are effective in providinga mechanism for releasing the drill bit from the remainder of the drillstring, the prior art devices are unduly complicated in design.

[0004] It is an object of the present invention to provide apparatusallowing the release of one part of equipment from another part thereof,wherein said apparatus is of a relatively simple design and relativelyinexpensive to manufacture.

[0005] The present invention provides downhole apparatus comprising afirst part connected to a second part by connecting means, theconnecting means limiting axial and rotational movement of the firstpart relative to the second part and comprising a connecting memberhaving a first end connected to said first part so as to limit axial androtational movement of said first part relative to said first end andhaving a second end connected to said second part so as to limit axialand rotational movement of said second part relative to said second end,wherein the connecting member is provided between said first and secondends with a region of weakness in the connecting member provided byweakening means, and with means for minimising stress generated in theconnecting means at said region of weakness when torque is transmittedin use between the first and second parts via the connecting member.

[0006] Thus, in use of downhole apparatus according to the presentinvention, the first part of the apparatus may be connected to a drillbit and a second part of the apparatus may be connected to a motor.Since the connection between the first and second parts is such as tolimit axial and rotational movement of said parts relative to oneanother, the drill bit may be suspended from the motor by means of theapparatus as the drill bit is run downhole. Furthermore, torque may betransmitted from the motor to the drill bit via the apparatus during adrilling operation. In the event that the drill bit becomes jammed androtational or axial movement of the remaining drill string is prevented,the drill string may be placed in tension so that the connecting memberbreaks at said region of weakness. Although the drill bit remains jammedinhole, the remainder of the drill string may then be retrieved. Theregion of weakness is such that the drill bit may be released withoutthe need to place the drill string under excessive tension. However, theregion of weakness is not so weak that normal axial movement of thedrill string cannot be undertaken without release of the drill bit.Despite the region of weakness in the connecting member, the apparatusof the present invention is capable of transmitting the high torquesassociated with drilling operations due to the means for minimisingstress generated in the connecting member at the region of weakness.

[0007] It is preferable for the connecting member to comprise an axiallyprojecting portion extending from said first end to said second end. Atleast one of said ends of the connecting member is provided with hookmeans for hooking about one of said first and second parts so as toallow a transmission of tension between said one part and the connectingmember. The hook means may comprise a portion extending perpendicularlyto the axially projecting portion. Said perpendicularly extendingportion may extend circumferentially about the longitudinal axis of theapparatus. Each end of said axially projecting portion of the connectingmember may be provided with hook means. The or each hook means may belocated adjacent a shoulder provided in one of the first and secondparts. Preferably, the connecting member is of an H-shape wherein thecross member of the H-shape is the axially projecting portion.

[0008] It is further preferable for the weakening means to be areduction in cross-sectional area of said axially projecting portion ofthe connecting member. Ideally, the weakening means is an apertureprovided in the axially projecting portion. The connecting means maycomprise a plurality of connecting members as described above. It ispreferable for the connecting means to comprise four of said connectingmembers.

[0009] Furthermore, said means for minimising stress generated in theconnecting member at said region of weakness comprises a shoulderdefined by one of said parts, wherein the shoulder comprises an axiallyextending surface for abutting the, connecting member in said region ofweakness during transmission of torque through said apparatus when inuse. The axially extending surface of said shoulder preferably abuts theconnecting member in a region remote from the region of weakness.

[0010] Embodiments of the present invention will now be described withreference to the accompanying drawings in which:

[0011]FIG. 1 is a side view of a first embodiment of the presentinvention (shown without a cover sleeve);

[0012]FIG. 2 is a cross-sectional side view of the embodiment shown inFIG. 1 (shown with a cover sleeve secured in position);

[0013]FIG. 3 is a cross-sectional side view of first and second parts ofthe first embodiment;

[0014]FIG. 4 is a plan view of a connecting member of the firstembodiment;

[0015]FIG. 5 is an end view of four of the connecting members of FIG. 4arranged in a circular configuration as shown in FIGS. 1 and 2;

[0016]FIG. 6 is a side view of a second embodiment of the presentinvention (shown without a cover sleeve);

[0017]FIG. 7 is a cross-sectional side view of the embodiment shown inFIG. 1 (shown with a cover sleeve secured in position);

[0018]FIG. 8 is a cross-sectional side view of first and second parts ofthe second embodiment;

[0019]FIG. 9 is a plan view of a connecting member of the secondembodiment; and

[0020]FIG. 10 is an end view of four of the connecting members of FIG. 4arranged in a circular configuration as shown in FIGS. 1 and 2.

[0021] A first downhole release joint 2 is shown in FIGS. 1 and 2 of theaccompanying drawings. The release joint 2 comprises first and secondparts 4,6 which are each of a generally cylindrical shape ofapproximately the same external diameter. The first part 4 has anexternal screw thread 8 for engagement with downhole equipment such as amotor and the second part 6 has an internal screw thread 10 forengagement with downhole equipment such as a drill bit. The end of thesecond part 6 distal to the internal screw thread 10 has a reducedexternal diameter which locates within the end of the first part 4distal to the external screw thread 8. The first part 4 abuts an annularexternal shoulder 12 defined on the external surface of the second part6. The two parts 4,6 of the release joint 2 locate in abutment with oneanother so as to define a substantially cylindrical assembly having anaxially projecting bore extending therethrough.

[0022] The external surface of each part 4,6 is provided with acircumferentially extending slot 16,18 (see FIG. 3 in particular) andfour equi-spaced and axially projecting slots 20 extending from saidcircumferentially projecting slot 16,18. As can be seen in FIGS. 1 to 3,the axially projecting slots 20 are arranged so that, when the first andsecond parts 4,6 are engaged in abutment with another, said axial slots20 of one part 4 align with those axial slots 20 of the other part 6 soas to define a total of four axially projecting slots extending betweenthe circumferentially projecting slots 16,18.

[0023] The two parts 4,6 of the release joint 2 are retained in abutmentwith one another by means of four identical connecting members 22 (seeFIG. 4). Each connecting member 22 is of an H-shape sized to locate inthe circumferentially and axially projecting slots 16,18,20. The crossbar 24 of the H-shape locates in a pair of aligned axially projectingslots 20. The portions 26 of connecting member 22 either end of thecross bar 24 have a curved part cylindrical shape (as most clearly seenin FIG. 5) for locating in the circumferentially projecting slots 16,18.It will be seen by reference to FIG. 1 in particular that the relativedimensions of the connecting members 22 and circumferentially/axiallyprojecting slots 16,18,20 is such that, with the connecting members 22located in said slots, relative axial and rotational movement betweenthe first and second parts 4,6 is, minimal. In this regard, it ispreferable for the cross bar 24 to locate within aligned axial slots 20with an interference fit. In this way, relative rotation between thefirst and second parts 4,6 is reduced to a minimum. Similarly, thelength of each pair of aligned axial slots 20 relative to the distancebetween end portions 26 of an associated connecting member 22 is suchthat said end portions locate within the circumferentially projectingslots 16,18 with an interference fit. In this way, relative axialmovement of the first and second part, is reduced to a minimum.

[0024] The cross bar 24 is provided with an aperture 28 through thethickness thereof. The aperture 28 is provided between the end portions26. In the assembled release joint 2, the aperture 26 locates in aregion of axially projecting slot 20 defined by one only of said parts4,6. The aperture 28 does not span the two parts 4,6.

[0025] With reference to FIGS. 1 and 5, it will be seen that the crossbar 24 of each connecting member 22 is upstanding for reception withinan axially extending groove defined on the interior surface of a coversleeve 30 (shown in FIG. 2). The cover sleeve 30 is a cylindrical memberhaving an external diameter substantially equal to the external diameterof the portion of the second part 6 located adjacent the internal thread10. The cover sleeve 30 locates in abutment with an external shoulder 32provided on the exterior surface of the second part 6. The cover sleeve30 is retained in position by means of four equi-spaced screws 34 (onlytwo of which are visible in FIG. 2). Prevention of rotational movementof the cover sleeve relative to the first and second parts 4,6 isfurther assisted by the reception of the upstanding cross bars 24 of theconnecting members 22 within the aforementioned internal cover sleevegrooves. If required, the external surface of the cover sleeve 30 may beprovided with blades and/or stabiliser fins. A fluid seal between thecover sleeve 30 and the first and second parts 4,6 is ensured by meansof two O-ring seals 36,38. These O-ring seals 36,38 locate incircumferential grooves 40,42 defined in the first and second parts 4,6respectively. Adequate retention of the connecting members 22 is alsoassisted by means of a circlip 44.

[0026] During use, the external thread 8 may be used for engagement witha motor and the internal screw thread 10 may be used for engagement witha drill bit. Thus, as the drill bit is run downhole, the weight thereofis suspended from the motor placing the cross bar 24 of each connectingmember 22 in tension. Despite the region of weakness in each cross bar24 created by each aperture 28, the connecting members 22 collectivelyhave sufficient tensile strength to allow maneuvering of the drill bitwithout premature separation of the two parts 4,6. In the event that thedrill bit becomes jammed in such a way that retrieval of the drillstring from a downhole position is prevented, uphole force may beapplied to the drill string so as to increase tension within each crossbar 24 to the extent that each cross bar 24 breaks at its aperture 28.The first part 4 portion of drilling string attached thereto may then beremoved from the hole leaving only the drill bit and second part 6 (andcover sleeve 30) of the release joint 2. The drill bit and first part 4may then be retrieved with specialist retrieval equipment.

[0027] Although the cross bar 24 of each connecting member is providedwith weakening means in the form of an aperture 28, the release jointremains capable of transmitting high torque loads. This is possible byvirtue of the support provided to the cross bar 24 by the sides of theassociated axially projecting slot 20 provided in the second part 6. Itwill be understood that, when a motor applies torque to the second part6 of the release joint 2, this torque is transmitted via the sides ofthe axially projecting slot 20 to a length of cross bar 24 remote to theaperture 28. In this way, the region of cross bar 24 provided with theaperture 28 is not subjected to undesirably high stress during torquetransmission which may result in the cross bar 24 breaking. Torque istransmitted from the first part 4 to the second part 6 by means of aportion of cross bar 24 capable of withstanding the stresses involved.

[0028] The present invention is not limited to the specific embodimentdescribed above. Alternative arrangements will be apparent to a readerskilled in the art For example, in the release joint 2 of FIGS. 1 to 5,the end of the second part 6 distal to the internal screw thread 10 isprovided with a GS external profile. Once the first part 4 has beenremoved from the wellbore with the remainder of the drill string, the GSprofile may be latched onto a fishing tool. Since the GS profile is onlysuitable for transmitting axial forces to the second part 6 and drillbit, use of the first downhole release joint 2 is particularly suited tooperations employing coil strings. However, in an alternative secondembodiment as shown in FIGS. 6 to 10, a second downhole release joint 50is shown wherein the second part 6 is provided with an external standardAPI thread profile rather than a GS profile. It should be noted that thefirst part 4 is not provided with a cooperating internal screw thread.Thus, once the first part 4 has been pulled from the second part 6 andpulled uphole, the jammed drill bit may be threadedly engaged with afishing tool by virtue of the aforementioned API threaded pin-upprojection. The connection provided by this threaded pin-up allows thetransmission of torque to the jammed drill bit. Components of the secondrelease joint 50 common with the first release joint 2 are identified inthe accompanying drawings with life reference numerals.

[0029] In the embodiments shown, the connecting members 22 are arrangedto break under between 60000 to 80000 LBS tension and 12000 FT LBS TYPtorsion.

[0030] Further alternative arrangements will be apparent to the skilledreader.

1. Downhole apparatus comprising a first part connected to a second partby connecting means, the connecting means limiting axial and rotationalmovement of the first part relative to the second part and comprising aconnecting member having a first end connected to said first part so asto limit axial and rotational movement of said first part relative tosaid first end and having a second end connected to said second part soas to limit axial and rotational movement of said second part relativeto said second end, wherein the connecting member is provided betweensaid first and second ends with a region of weakness in the connectingmember provided by weakening means, and with means for minimizing stressgenerated in the connecting means at said region of weakness when torqueis transmitted in use between the first and second parts via theconnecting member.
 2. Downhole apparatus as claimed in claim 1, whereinthe connecting member comprises an axially projecting portion extendingfrom said first end to said second end.
 3. Downhole apparatus as claimedin claim 1 [[or 2]] wherein at least one of said ends of the connectingmember is provided with hook means for hooking about one of said firstand second parts so as to allow a transmission of tension between saidone part and the connecting member.
 4. Downhole apparatus as claimed inclaim 3, wherein the hook means comprises a portion extendingperpendicularly to the axially projecting portion.
 5. Downhole apparatusas claimed in claim 4, wherein said perpendicularly extending portionextends circumferentially about the longitudinal axis of the apparatus.6. Downhole apparatus as claimed in any or claims 3 to claim 5, whereineach end of said axially projecting portion of the connecting member isprovided with hook means.
 7. Downhole apparatus as claimed in any ofclaims 3 to claim 6, wherein the or each hook means is located adjacenta shoulder provided in one of the first and second parts.
 8. Downholeapparatus as claimed in any of claims 2 to claim 7, wherein theconnecting member is of an H-shape wherein the cross member of theH-shape is the axially projecting portion.
 9. Downhole apparatus asclaimed in any of claims 2 to claim 8, wherein the weakening meanscomprises a reduction in cross-sectional area of said axially projectingportion of the connecting member.
 10. Downhole apparatus as claimed inclaim 9, wherein the weakening means comprises an aperture provided inthe axially projecting portion.
 11. Downhole apparatus as claimed inclaim 1, wherein the connecting means comprises a plurality ofconnecting members as claimed in any of the preceding claims. 12.Downhole apparatus as claimed in claim 1, wherein the connecting meanscomprises four connecting members claimed in any of the precedingclaims.
 13. Downhole apparatus as claimed in any of the preceding claimsclaim 1, wherein said means for minimizing stress generated in theconnecting member at said region of weakness comprises a shoulderdefined by one of said parts, the shoulder comprising an axiallyextending surface for abutting the connecting member in said region ofweakness during transmission of torque through said apparatus when inuse.
 14. Downhole apparatus as claimed in claim 13, wherein the axiallyextending surface of said shoulder abuts the connecting member in aregion remote from the region of weakness.
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